The following correction was printed in the Guardian's Corrections and Clarifications column, Friday October 10 2003

The subhead of this article stated, if Cardinal Francis Arinze should succeed Pope John Paul II, he would be the first African pope. The text made it clear that he would be "the first African in 1,500 years to sit on the throne of St Peter". Three Africans have already occupied the office, according to the Catholic Encyclopedia: Victor (around AD 183-203), Mechiades or Militiades (possibly AD 311-314), and Gelasius (AD 492-496).

The Vatican has a thing about colour. There is a white cassock for the pontiff; scarlet cloaks for the cardinals; navy and gold uniforms for the Swiss Guards; yellow for the flag - and soon, we might know whether there is white smoke for a black pope.

Behind the dark shutters overlooking St Peter's Square, the 83-year-old leader of the Roman Catholic church is ailing, possibly dying. His aides seem to be preparing the public with solemn pronouncements saying he is in a very bad way, prompting some in the crowds gathered beneath his window this week to weep.

Others cast their eyes to the right of the apostolic palace, to the window of a cardinal who could become very famous very soon. Francis Arinze is tipped by some to succeed John Paul II and become the first African in 1,500 years to sit on the throne of St Peter.

If he does, the world will devour every detail about this stocky Nigerian - that he spent his early years in the countryside outside the Catholic faith; that he loves tennis and football; that he hangs African masks on his apartment walls; that he thinks Muslims, Buddhists and Jews can go to heaven; and that he stopped taking sugar in his tea during the Biafran war.

For many, the most compelling detail would be his race: a black man with more international influence than many white prime ministers and presidents. As pope he would wield a unique blend of spiritual and political power, able to mobilise the opinions of up to a billion Catholics and sway the policies of nation states.

It has been quite a journey from the baked red earth of Eziowelle, a village in south-east Nigeria, to the marbled halls of Rome, where Cardinal Arinze, 70, is the prefect of the congregation for divine worship and the discipline of the sacraments, a post which puts him in charge of liturgy, and number four in the Vatican hierarchy.

"It has been a meteoric rise," said Gerard O'Connell, a Vatican analyst who this week published a book of interviews with the cardinal, called God's Invisible Hand. "The guy is is very bright and astute and able to communicate in simple language. He has a great sense of joy."

Cardinal Arinze was first touted as papabile - "popeable" - by another Vatican analyst, Peter Hebblethwaite, in 1992. Since then he has stayed on the notional shortlist of successors, seldom putting a foot wrong in the writings, speeches and diplomacy by which cardinals are evaluated.

Any open backing for a candidacy is frowned upon in as much as it anticipates the incumbent's death. However, Cardinal Joseph Ratzinger, a confidant of John Paul and the Vatican's theological watchdog, came close when he said electing an African pope "would be a positive sign for the whole of Christendom".

"For all its condemnation of racism, the western world still has reservations about the third world," Cardinal Ratzinger said. "Yet, in Africa for example, we have truly great figures whom we can only admire. They are fully up to the job." Two Latin American cardinals recently swelled that chorus.

John Allen, the author of a book about the next papal election, Conclave, said a recurring theme in their interviews with 45 cardinals was a desire for a pope from the developing world - "which could mean Arinze. To be frank, there are not many other Africans".

However, Mr Allen does not rank the Nigerian in the top 5 - only in the top 20 - partly because there are strong Latin American candidates, and partly because there is a desire by some Italians to "reclaim" a job which they see as being "lent" to a Pole after centuries of Italians.

Austen Ivereigh, the deputy editor of the Catholic weekly the Tablet, also puts Cardinal Arinze in the top 20.

John Paul's successor will be elected by a conclave of 135 cardinals aged under 80. Gathered in the Sistine chapel, they will each write a name on a card, kneel at the altar, and slide the ballot into a chalice. A two-thirds majority is needed, so there can be many rounds before a consensus emerges.

Cardinal Arinze's geniality could tip the balance, said Mr Ivereigh: "He is not known to get up people's noses."

White smoke pluming from the chapel will signal that the choice has been made, and a short while later - his white cassock adjusted by Rome's Gammarelli tailors - the new pope will appear at the basilica window to greet the crowds. If it turns out to be Cardinal Arinze, it will compel universal recognition that two thirds of Catholics live below the equator, and that Europeans no longer dominate the church.

Born on November 1 1932 - All Saints' Day - Francis Arinze was the third of seven children. His parents, Joseph and Bernadette, worshipped traditional Ibo deities, but sent Francis to an Irish missionary school. By the age of nine he had decided to be baptised and set out on the path to priesthood.

For his ordination, he sailed to Liverpool in 1955 and caught a train to Rome, carrying his luggage on to the No 64 bus to the Vatican. He won top grades for his doctoral thesis on sacrifice in the Ibo religion, foreshadowing his future effort to bring elements of African tradition into Catholic services.

He studied teaching in London in 1963 and returned to Nigeria two years later to become the world's youngest bishop, aged 32. After attending the second Vatican council, he returned home to a civil war between Nigeria and Biafran secessionists which had killed a million people by 1970.

In his interviews with Mr O'Connell, Cardinal Arinze recalls being on the run from Nigerian forces, witnessing horrors and sheltering from falling bombs. "He concluded that war makes problems more acute, it doesn't solve them," said Mr O'Connell.

Cardinal Arinze presided over a boom in the numbers working for the church, and annoyed some by insisting his priests could only drive modest cars. John Paul was so impressed when he visited in 1982 that he summoned him to Rome to manage the Vatican's relations with other faiths.

Shuttling to synagogues, mosques and temples, he forged ties with other religions while maintaining the Pope's strict line on doctrine.

"The beautiful thing about the cardinal is that he can say the hardest thing with a smile on his face and not offend people," said one colleague.

If "handling" communism was the Vatican's 20th-century challenge, Islam is widely thought to be this century's, and one which Cardinal Arinze is equipped to face.

"He is a popular fellow. He makes you laugh, he doesn't stand on ceremony, he answers his own phone, and he's comfortable with women," said one woman on the diplomatic circuit.

But there are some who say Cardinal Arinze is an intellectual lightweight - that he parrots John Paul and would be out of his depth with world leaders. "He is seen as not especially creative, an unoriginal thinker," said Mr Allen.

But Ian Linden, a former director of the Catholic Institute for International Relations, said it was not necessary for a pope to be an outstanding intellectual - "especially since Arinze has been careful to keep highly skilled intellectuals around him".

A more serious concern is his political antennae. Some audiences have given his words standing ovations, but others have been been infuriated and bewildered. There were protests at Georgetown University in the US when he seemed to equate homosexuality with pornography, fornication and adultery. In a lesser-known incident in London, he insisted that altar girls should remain immobile during mass.

But such conservatism wins support from potential kingmakers such as Cardinal Ratzinger, while his championing of human rights, debt relief and tolerance for other religions plays well with some progressives.

In the book of interviews published this week, Cardinal Arinze says: "I will not manoeuvre, I will not do politicking, I will not try to arrange my future."

Career: Ordained 1958; consecrated bishop (Africa's youngest) 1965; first black archbishop of Onitsha 1967; called by John Paul II to Vatican in 1984; elevated cardinal 1985; president of Pontifical Council for Inter-religious Dialogue 1984-2002; prefect of divine worship and discipline of sacraments, 2002 to date.

Arinze on family: 'In many parts of world, the family is under siege, opposed by an anti-life mentality as is seen in contraception, abortion, infanticide and euthanasia. It is scorned and banalised by pornography, desecrated by fornication and adultery, mocked by homosexuality, sabotaged by irregular unions, and cut in two by divorce.'